For every BMI unit above 29 kg/m2, the probability of spontaneous pregnancy is reduced by approximately 4% (HR: 0.96, 95% CI: 0.91 to 0.99), compared with a reference group of women who were not overweight, reported Dr. Jan Willem van der Steeg, of University Medical Center here, and colleagues in the Dec. 12 issue of Human Reproduction.

"This reduction in fertility is comparable to an increment of one year in female age," Dr. van der Steeg noted. The researchers pointed out that approximately 12% of women in Western Europe and 25% of women in North America of childbearing age are obese. "Given the increased prevalence of obesity, this is a worrying finding," Dr. van der Steeg said.

While obesity is known to be a risk factor for anovulation, and is associated with a reduced likelihood of conception, the relationship in women who are ovulatory was unestablished.

So the researchers consecutively enrolled 3,029 subfertile couples from January 2002 through February 2004. The study took place in 24 hospitals in the Netherlands.

Each couple underwent a fertility work-up which included a history of fertility, details about height and weight, smoking habits, assessment of tubal patency, ovulation, semen analysis, the ages of both partners, and the length of subfertility, which was defined as secondary if a woman had conceived before. All women had to have at least one patent fallopian tube, and their partners had to have normal semen analyses.

The primary study endpoint was time to conception without treatment. Couples were followed from the time of work-up to pregnancy or the initiation of fertility treatment within 12 months.

Weight in kilograms divided by the square of the height in meters, both self-reported during the initial visit, were used to calculate BMI: 3.7% of the women were below a BMI of 18.5; 67% were between 25-30; 6.7% were between 30 and 35; and 3.8% had a BMI of >35. Women in the reference group had a BMI of between 21 and 29 kg/m2.

After 12 months, 20% of couples had achieved a spontaneous pregnancy. Of those who did not, 38% had already begun fertility treatment.

On multivariate analysis -- which adjusted for duration of subfertility, female age, referral status, previous pregnancy, semen motility, and current smoking status of both partners -- the results remained unchanged: HR: 0.96 per kg/m2 (95% CI: 0.91 to 0.99).

Notably, women who were obese, defined as a BMI of 35 kg/m2, had a 26% reduced chance of becoming pregnant, and for women with a BMI of 40 kg/m2, their chances of conceiving were reduced by as much as 43%, compared with the reference group.

While the researchers are not yet sure of the pathophysiology involved, they speculated that the protein hormone leptin, which affects body weight, metabolism, and reproductive function, could be involved.

They acknowledged several limitations of their study, including the fact that frequency of sexual intercourse was not taken into account, nor was the BMI of the male partner.

They pointed out that " a review found support that obesity is associated with decreased intercourse frequency, reduced sexual desire, and erectile dysfunction. However, in view of the paucity of data, confounding factors,

like medication and adverse lifestyles, could not be ruled out."

Currently, the researchers are developing a prediction model for pregnancy that involves BMI and other factors. However, the original prediction model, which does not include BMI, is available on the internet at http://www.freya.nl/probability.php, and can be used to help physicians and their patients make decisions concerning fertility treatment.

This study was funded by ZonMW, The Netherlands Organization for Health Research and Development, The Hague, The Netherlands.

The authors made no declarations on financial conflicts of interest.

Reviewed by Zalman S. Agus, MD Emeritus Professor University of Pennsylvania School of Medicine

Accessibility Statement

At MedPage Today, we are committed to ensuring that individuals with disabilities can access all of the content offered by MedPage Today through our website and other properties. If you are having trouble accessing www.medpagetoday.com, MedPageToday's mobile apps, please email legal@ziffdavis.com for assistance. Please put "ADA Inquiry" in the subject line of your email.